The present invention relates to a scrubber system for removing carbon dioxide from a metal-air or fuel cell battery.
More particularly, the present invention relates to such a scrubber system and to a method for maximizing the effective life and utilization of carbon dioxide-absorbent material in such a scrubber system.
As is known, and as described, e.g., in U.S. Pat. No. 3,909,206, alkali electrolyte fuel cells and metal-air batteries require a clean fuel and a clean oxidant to generate power. Many oxidants, such as air and all but the most pure, and therefore the most expensive, oxygen supplies and some fuels contain carbon dioxide (CO.sub.2) which, when brought into contact with the electrolyte, combine with the electrolyte to form carbonates. The formation and presence of carbonates in the electrolyte decreases the voltage of the cells and batteries, and eventually causes their failure.
For this reason, a CO.sub.2 scrubber system is applicable to a metal-air or fuel cell battery (e.g., zinc-air battery, hydrogen-air fuel cell) with an alkaline electrolyte (e.g., aqueous KOH) and incorporating an air electrode, supplied with air as the cathodic reactant. Regular air contains about 400 ppm of CO.sub.2, and if this CO.sub.2 is not removed, the CO.sub.2 can react with the KOH to form potassium carbonate (K.sub.2 CO.sub.3), which will gradually build up in the alkaline electrolyte. EQU 2KOH+CO.sub.2 =K.sub.2 CO.sub.3 +H.sub.2 O (1)
K.sub.2 CO.sub.3 not only reduces the conductivity and alkalinity of the KOH, giving poorer cell polarization characteristics, but, being less soluble than KOH, can deposit carbonate crystals in the pores of the air electrode, especially in the presence of other sparingly soluble ions, such as zincares and aluminates in the electrolytes of zinc-air and aluminum-air cells respectively. These crystals can cause leaks and shorten the life of air electrodes.
The general absorption reaction of CO.sub.2 by alkali metal hydroxide (MOH) may be written: EQU 2MOH+CO.sub.2 =M.sub.2 CO.sub.3 +H.sub.2 O (2)
The prior art has therefore suggested the use of a scrubber system containing alkali hydroxide granules, held as a compact bed in a suitable container whose dimensions, the granule size, the granule loading and degree of packing of the granules are determined by such factors as the required air flow rate and flow time through to the battery, permitted pressure drop across the granule bed, permitted CO.sub.2 exit concentration and degree of absorption required within the bed. Advantageously, the granules are retained between plastic screens somewhat finer than the granule dimensions. When the bed is no longer effective for scrubbing and CO.sub.2 breakthrough occurs, as shown, for example, visually (by a color change of a chemical indicator impregnated on the granules, which signals chemical exhaustion of the bed) or electronically (by the output of a CO.sub.2 detector, for example of the infra-red type, showing CO.sub.2 levels above a certain predetermined value, for example, 50 ppm), the bed must be replaced.
There is therefore a need for a scrubber system of special application to serviceable batteries of the above type (e.g., mechanically-rechargeable zinc-air) wherein there is a need to periodically maintain the CO.sub.2 scrubber system, and it is desirable, for economical and/or ecological reasons, to reprocess the spent scrubber active material rather than simply disposing of spent scrubber material and using of fresh material.
U.S. Pat. No. 3,909,206 teaches a scrubber using finely-ground alkali hydroxide particles mixed with fine particles of a hydrophobic material, such as polytetrafluoroethylene, for removing carbon dioxide from a gas stream to a concentration of less than 0.25 ppm.
Although CO.sub.2 levels may be reduced to less than 0.25 ppm, no means are provided for reprocessing the scrubber material, or for extending the effective life thereof.
U.S. Pat. No. 3,990,912 for hydrogen-air fuel cells with alkaline electrolytes uses electrochemical means to convert K.sub.2 CO.sub.3 in the cell electrolyte back to KOH, by means of an additional regenerator cell system with circulating electrolyte that consumes hydrogen when it operates. This may be too complex, heavy and parasitic as to power needs for a mobile system application (e.g., an electric vehicle) and, requiring a source of hydrogen, will not be applicable to non-hydrogen systems (e.g., aluminum-air).
U.S. Pat. No. 4,047,894 describes a scrubber element comprised of spaced-apart corrugated layers of porous PVC impregnated with CO.sub.2 -absorbing solution (e.g., 10 Moles/liter aqueous KOH). Although means for reuse of the element are described (column 3, lines 49-52), comprising rinsing with water, drying and reimpregnation with absorption solution, no means are given for reprocessing of the spent absorber to give fresh absorbent free of carbonate deposits, or for extending the effective life thereof by mechanical means.
In light thereof, there is now provided, according to the present invention, a method for maximizing the effective life and utilization of CO.sub.2 -absorbent material in a scrubber system for removing carbon dioxide from an air inflow to a metal-air battery or fuel cell, comprising providing in such a battery a housing including a gas inlet, a gas outlet and at least one removable, gas-permeable container containing a CO.sub.2 -absorbent material, said container being positioned across the flow path of gas entering said inlet and exiting said outlet; wherein, after a predetermined utilization of said system, said at least one removable, gas-permeable container is removed from said system and spent CO.sub.2 -absorbent material from said container is regenerated for reuse in said scrubber system, said regeneration including at least periodically removing accumulated Group 1a metal carbonate deposits.
In preferred embodiments of the present invention, said CO.sub.2 -absorbent material comprises granules of a Group 1a metal hydroxide or a hydrate thereof, and wherein regeneration thereof comprises the steps of:
a) removing spent CO.sub.2 -absorbent material from said container and comminuting said material; PA1 b) thermally decomposing said material; PA1 c) hydrolysing the material from step (b) with water to reform the hydroxide which is obtained in the solid phase, by means of crystallizing out and drying; PA1 d) comminution and granulation of the hydrated material to predetermined particle size and porosity; and PA1 e) repacking the regenerated particles in a scrubber container for reuse.
In other preferred embodiments of the present invention, said CO.sub.2 -absorbent material comprises a solution of Group 1a metal hydroxide in water absorbed into porous granules of an alkali-resistant material, and wherein regeneration thereof comprises immersing the scrubber container in a flowing stream of a Group 1a metal hydroxide-rich solution until all the absorbent carrier granules have picked up fresh Group 1a metal hydroxide solution, and draining the container of excess solution, whereafter said container is ready for reuse.
In these later embodiments, Group 1a metal carbonate accumulates in said regeneration solution after several uses thereof, and thus the present method comprises the further step of slaking said regeneration solution with lime or barium hydroxide to remove accumulated Group 1a metal carbonate deposits therefrom, followed by filtering off insoluble carbonates.
When said Group 1a metal hydroxide is NaOH, the present method may alternatively comprise the further step of chilling the solution to at least -10.degree. C., whereupon sodium carbonate settles out, followed by filtering off said sodium carbonate.
It has now also been found, especially for scrubbers in mobile applications, that at CO.sub.2 breakthrough there still remains a substantial amount of granules in the scrubber bed that has hardly reacted, with most of the reaction having occurred in an outer layer through which air enters the bed. If the complete bed is replaced at this stage, poor utilization of the overall scrubber material is realized.
In light thereof, the present invention now also provides a method for maximizing the effective life and utilization of carbon dioxide-absorbent material in a scrubber system for removing carbon dioxide from a metal-air battery or fuel cell, comprising providing, in such a battery, a housing including a gas inlet and a gas outlet, and a plurality of gas-permeable containers supported in series across the flow path of gas entering said inlet and exiting said outlet, each of said containers containing a carbon dioxide-absorbent material and being individually independently removable from said series, wherein, after a predetermined utilization of said system, a first container closest to said gas inlet is removed from said series and a second container from said series is repositioned in the place of said first container, while the material in said first container is regenerated as described herein.
As will now be understood, in this embodiment of the present invention, the scrubber bed is sub-compartmentalized into two or more layers. When breakthrough occurs, only the air-entry layer is removed, with means available (e.g., sliding, rotational, spring, levers, gravitational, hydraulic, pneumatic, manual, motorized, etc.) for repositioning the remaining layer(s) to take the place of a removed layer within the scrubber, and such that a new layer may be introduced into the air-exit side of the scrubber.
In the case of a zinc-air battery, where mechanical refueling of the zinc is required, for example, once per week, the scrubber material would be conveniently subdivided into three equal layers, only one of which is replaced at weekly intervals for reprocessing, and by this means only scrubber material with a high level of chemical conversion (above 80%) need be used.
Thus, in especially preferred embodiments of the present invention, there is provided a method wherein said scrubber is sub-compartmentalized into a series of three separately removable and repositionable gas-permeable containers, wherein, upon removal of a first container, a second container is repositioned in its place, whereafter a third container from said series is repositioned in place of said second container, and a new container, containing fresh carbon dioxide-absorbent material, is positioned in place of said third container.
In another embodiment of the present invention, after partial utilization of the CO.sub.2 -absorbent material, air feed conduits connected to said inlet and said outlet are manually disconnected by the user and exchange-reconnected thereto, for maximum utilization of the CO.sub.2 -absorbent material before the regeneration thereof as described herein.
In yet another embodiment of the present invention, there is provided a removable, tubular, gas-permeable container containing a CO.sub.2 -absorbent material between concentric inner and outer cylindrical walls thereof and creating a flow path from said inner to said outer wall, wherein the area of CO.sub.2 -absorbent material increases along said flow path.
The scrubber system of the present invention is preferably based on the use of a coarse, granular type CO.sub.2 -absorbent material (3-30 mesh), comprising an alkali metal hydroxide (e.g., selected from LiOH, NaOH, KOH). The hydroxide may be in the solid phase (either anhydrous or hydrated form), when advantageously a certain minimum porosity (50% minimum) of the granules will ensure good utilization of the inner portions of said granules. Alternatively, especially when the air to the scrubber/ battery is prehumidified, the hydroxide may be in the form of an impregnated phase as an aqueous solution (e.g., 30-40 wt. % MOH) absorbed on porous carrier granules of an alkali-resistant plastic, ceramic or elastomer in foam, flock, chip or felt form. Examples of such materials are polyethylene, polypropylene, PVC, polystyrene, nylon, low-density brick, or rubber.
If an aqueous solution of alkali metal hydroxide is used to impregnate the carrier granules and the carrier is sufficiently porous (at least 50%), the excellent hygroscopic properties of the hydroxide will ensure under humidified air an effective, quasi-liquid phase at the granule surface, enabling adequate use of the sub-surface layers as well for CO.sub.2 absorption.
For regeneration of the scrubber material, there are two approaches, depending upon whether the scrubber material is used in the solid phase (preferred for LiOH-based scrubbers), or as a solution phase impregnated on a porous carrier (preferred for NaOH, KOH-based scrubbers). If solid phase, the spent scrubber material (mainly the carbonate M.sub.2 CO.sub.3), after removal from the scrubber container, may be comminuted and then decomposed thermally to give the metal oxide M.sub.2 O, usually requiring a high-temperature roast step (900.degree.-1400.degree. C.): EQU M.sub.2 CO.sub.3 =M.sub.2 O+CO.sub.2 ( 3)
The product M.sub.2 O may be hydrolyzed with water to reform the hydroxide, which is obtained in the solid phase by means of crystallization, drying and comminution, and may then be regranulated to give fresh solid MOH and repacked into the scrubber container. EQU M.sub.2 O+H.sub.2 O=2MOH (4)
Instead of using a thermal cycle, the spent M.sub.2 CO.sub.3 may be slaked with a low-cost reagent, such as lime (CaO) in a wet process to reform MOH, which is then obtained in the solid phase by separation of the precipitated CaCO.sub.3 from the aqueous MOH phase using, for example, filtration followed by evaporation/granulation. As an alternative to lime, barium hydrate is applicable. EQU M.sub.2 CO.sub.3 +CaO+H.sub.2 O=2MOH+CaCO.sub.3 ( 5) EQU M.sub.2 CO.sub.3 +Ba(OH).sub.2 =2MOH+BaCO.sub.3 ( 6)
The reactions (4), (5) are exothermic and the heat evolved may be usefully employed elsewhere. Additionally, the CO.sub.2 product from (3) or the CaCO.sub.3 /BaCO.sub.3 (precipitated) product from (5) or (6) are recoverable for resale or reprocessing back to CaO or Ba(OH.sub.2), while a chemical indicator for showing CO.sub.2 saturation may be added at the final granulation stage.
If the scrubber material is impregnated as a solution on a porous granular carrier, the element containing spent material is left soaking in a flowing excess closed cycle wash stream of low carbonate-MOH solution until the carbonate content in the granules has reached an acceptably low level, and after draining the element may be reused. The wash stream containing both MOH and rejected M.sub.2 CO.sub.3 is continuously or batch purified by either physical means (only for NaOH/Na.sub.2 CO.sub.3) by cooling to -10.degree. C. (when Na.sub.2 CO.sub.3 separates out by crystallization), or by chemical means (slaking with lime or barium hydrate as in Equation (5) or Equation (6) above).
The invention will be described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood.
With reference to the figures, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.